The present invention relates generally to a process for heat treating conductive polymer compositions and electrical devices to improve their electrical properties.
It is well known that the resistivity of many conductive materials change with temperature. Resistivity of a positive temperature coefficient (xe2x80x9cPTCxe2x80x9d) material increases as the temperature of the material increases. Many crystalline polymers, made electrically conductive by dispersing conductive fillers therein, exhibit this PTC effect. These polymers generally include polyolefins such as polyethylene, polypropylene, polyvinylidene fluoride and ethylene/propylene copolymers. Certain doped ceramics such as barium titanate also exhibit PTC behavior.
At temperatures below a certain value, i.e., the critical or switching temperature, the PTC material exhibits a relatively low, constant resistivity. However, as the temperature of the PTC material increases beyond this point, the resistivity sharply increases with only a slight increase in temperature.
Electrical devices employing polymer and ceramic materials exhibiting PTC behavior have been used as overcurrent protection in electrical circuits. Under normal operating conditions in the electrical circuit, the resistance of the load and the PTC device is such that relatively little current flows through the PTC device. Thus, the temperature of the device due to I2R heating remains below the critical or switching temperature of the PTC device. The device is said to be in an equilibrium state (i.e., the rate at which heat is generated by I2R heating is equal to the rate at which the device is able to lose heat to its surroundings).
If the load is short circuited or the circuit experiences a power surge, the current flowing through the PTC device increases and the temperature of the PTC device (due to I2R heating) rises rapidly to its critical temperature. At this point, a great deal of power is dissipated in the PTC device and the PTC device becomes unstable (i.e., the rate at which the device generates heat is greater than the rate at which the device can lose heat to its surroundings). This power dissipation only occurs for a short period of time (i.e., a fraction of a second), however, because the increased power dissipation will raise the temperature of the PTC device to a value where the resistance of the PTC device has become so high that the current in the circuit is limited to a relatively low value. This new current value is enough to maintain the PTC device at a new, high temperature/high resistance equilibrium point, but will not damage the electrical circuit components. Thus, the PTC device acts as a form of a fuse, reducing the current flow through the short circuit load to a safe, relatively low value when the PTC device is heated to its critical temperature range. Upon interrupting the current in the circuit, or removing the condition responsible for the short circuit (or power surge), the PTC device will cool down below its critical temperature to its normal operating, low resistance state. The effect is a resettable, electrical circuit protection device.
Devices having higher resistance in the tripped state, i.e., at its new, high temperature/high resistance equilibrium point, are useful for high voltage applications. However, often during the manufacturing process of PTC devices the polymer composition is exposed to high temperatures, mechanical shear, thermal gradients and other influences which affect the electrical properties of the polymer composition, and particularly lower the peak resistance of the device rendering it unacceptable for higher voltage applications. Additionally, the resistance of the device can be adversely affected when the device is soldered to a PC board, once again rendering the device unacceptable for specific applications.
The present invention is directed to a method of heat treating a polymer PTC composition to raise the peak resistivity of the material. By raising the peak resistivity of the material, an electrical circuit protection device employing the material will exhibit an increased resistance in the trip or fault state. Devices heat treated according to the present invention are especially well suited for high voltage applications.
In a first aspect of the present invention there is provided a method for heat treating a polymer PTC composition having a melting point temperature Tmp. In the first step, the temperature of the polymer PTC composition is increased at a first rate, r1, to a temperature greater than Tmp. The temperature of the polymer PTC composition is held at this elevated temperature (greater than Tmp) for a predetermined period of time. The temperature of the polymer PTC composition is then decreased to a temperature less than Tmp at a second rate, r2, wherein r2 is greater than r1.
In a second aspect of the present invention there is provided a method for heat treating a polymer PTC composition having an initial peak resistivity, Rpi, and a melting point temperature, Tmp. The method comprises the steps of increasing the temperature of the polymer PTC composition at a first rate, r1, to a temperature greater than Tmp. The temperature of the polymer PTC composition is held at this elevated temperature (greater than Tmp) for a predetermined period of time. Next, the temperature of the polymer PTC composition is decreased to a temperature less than Tmp at a second rate, r2, wherein r2 is greater than r1. After decreasing the temperature of the polymer PTC composition, the composition has a new peak resistivity, Rpn, which is at least 1.5xc3x97Rpi.